The evolution of modern oil and gas wells has led to increases in both the depth of the wells and the complexity of the procedures and equipment needed for drilling and completions operations. Additionally, there is an ongoing need for improved safety and efficiency in the drilling and completions process. The combination of these factors has created a need for improved visibility of the downhole conditions along the length of the drill string and at the bottom hole assembly (BHA) during drilling and completions operations. Downhole sensor measurements such as downhole bore and annular pressure, drill string torque and tension, and temperature can be transferred from a downhole location to the surface through one of several known telemetry methods.
One method of downhole communication is wired drill pipe telemetry, which offers very high bandwidths, but tends to be expensive to deploy and prone to failure. Another known downhole communication method is mud pulse telemetry which encodes sensor data into pressure waves that are induced in the drilling fluid flowing in the drill string. Drawbacks to mud pulse telemetry include an inability to transmit when drilling fluid is not flowing, and relatively low data rate transmissions which decrease as the depth of the well increases. A third method of downhole communication is electromagnetic (EM) telemetry, which transmits digitally modulated electromagnetic waves through the formations surrounding the drill string to a surface receiver. EM telemetry does not require the flow of drilling fluid and can provide a higher data transmission rate than mud pulse telemetry, but can be sensitive to the nature of the formations surrounding the well and may not be well suited for deeper wells.
A fourth method of downhole communication is acoustic telemetry, which has proven to be well suited for the modern drilling environment. Acoustic telemetry is capable of transmitting hundreds of bits per second, and since it uses the body of the drill pipe as its transmission medium, it is insensitive to the surrounding formation or casing, and does not require any fluid flow to enable the transmission of data.
There are currently three different implementations of acoustic telemetry systems in downhole tools that use acoustic telemetry: probe-based, clamp-on, and collar-based. These systems typically comprise components including sensors, electronics, batteries and an acoustic transmitter. The probe-based implementation is mounted at least partially within the bore of the drill pipe. The clamp-on implementation is mounted on the external wall of the drill pipe. The collar-based implementation places the components within an annular space in the downhole tool.
In a typical drilling or completions environment, a number of acoustic transmitters can be spaced along the length of the drill string. The most common type of acoustic transducer used within downhole tools comprises a cylindrical piezoelectric stack mounted in a collar-based implementation. Such a stack comprises a number of thin piezoceramic discs layered with thin electrodes between each disc which are connected electrically in parallel. As is known in the art, such as disclosed in U.S. Pat. No. 6,791,470, the entirety of which is incorporated by reference herein, an advantage of the piezoelectric stack when compared to other acoustic transducer types is that the acoustic impedance of the stacked ring structure can be closely matched to the acoustic impedance of the tool's structure thereby optimizing the transfer of acoustic energy from the stack into the tool body, and subsequently into the drill string. Any acoustic impedance mismatch between the stack and the tool surrounding structure results in a reduction in the acoustic output power of the tool.
The piezoelectric stack structure offers a large displacement force combined with a high energy conversion efficiency and high compressive strength, but offers little resistance to tension, even that incurred when voltage is applied. Due to its low tensile strength, it is common practice to place a piezoelectric stack under a mechanical compressive preload along the stack's axis of operation in order to maintain stack integrity while being actuated. The magnitude of the preload can compensate for dynamic forces, but also affects the mechanical energy output from the stack. If there is no compressive preload or if the compressive preload exceeds the blocking force of the piezoelectric material, then there is no mechanical energy output from the stack. An optimum preload level that will maximize the output mechanical energy from the stack occurs when the stiffness of the preloaded stack is equal to the stiffness of the mechanical load.
Referring to FIG. 1, a prior art collar-based piezoelectric stack-type acoustic transmitter 301 comprises first and second thermal expansion compensation rings 302a and 302b, a retaining ring 303, end coupling 304, a steel outer housing 305, a mandrel 306, a pin 307, and a piezoelectric stack 308. The first and second thermal expansion rings 302a and 302b are designed to compensate for the difference between the thermal expansion of the steel housing 305 and the piezoelectric stack 308. The mandrel 306 is threaded into the end coupling 304, and the first thermal expansion compensation ring 302a is slid down the mandrel 306 to an inner face 309 of the end coupling 304. The piezoelectric stack 308 is slid down the mandrel 306 to rest against the first thermal compensation ring 302a. The second thermal compensation ring 302b is slid down the mandrel 306 to rest against the end of the piezoelectric stack 308, and the retaining ring 303 is placed on the mandrel 306 against the second thermal compensation ring 302b. The outer housing 305 is placed over the mandrel 306, first and second thermal compensation rings 302a, 302b and the retaining ring 303 and threaded onto the end coupling 304. The pin 307 is threaded into the housing 305 until the thread is shouldered, and an inner face of the pin 310 is forced against the retaining ring 303 which in turn forces the thermal compensation rings 302a, 302b and the piezoelectric stack 308 against the immoveable inner face 309 of the end coupling 304, thereby creating a compressive preload force on the piezoelectric stack 308. The amount of compressive force on the piezoelectric stack can be controlled by varying the length of the retaining ring 303.
The prior art acoustic transmitter 301 will maintain a positive compressive preload on the piezoelectric stack 308 over a limited range of tension/compression on the downhole tool. However, in deeper wells such as those drilled offshore, the tension/compression applied to the downhole tool by external forces can result in the tool flexing enough to either reduce the preload to zero, or to compress the piezoelectric stack beyond its compressive limits. Thus there is a need for a method of applying a compressive preload to the piezoelectric stack in a downhole acoustic transmitter that will maintain an effective preload over the entire range of tension and compression applied to the downhole tool by the drill string while operating in a downhole environment.